The invention relates to photographic elements, such as colour negative films, and in particular to those which contain a development inhibitor-releasing (DIR) coupler in association with an image dye-forming coupler, hereinafter referred to as an image coupler, and more particularly to those wherein the DIR coupler has low reactivity compared to the image coupler.
Many silver halide photographic elements, in particular colour negative films, contain development inhibitor-releasing couplers. DIR couplers release inhibitors that can restrain silver development in the layer in which release occurs, as well as in other layers of a multilayer photographic material. DIR couplers can help control photographic gamma (contrast), enhance sharpness (acutance), reduce granularity, improve exposure latitude without speed loss and they can provide colour correction via interlayer interimage effects. A specific class of DIR couplers is DIAR couplers (development inhibitor anchiomeric-releasing) in which the release of the inhibitor is controlled by a timing mechanism.
Generally the DIR is incorporated in the photographic material at a much lower concentration than the image coupler, generally 5-10%. EP-A-0 867 763A describes DIRs in an element containing silver chloride or silver bromoiodide. JP-4278942 exemplifies silver iodide and silver bromoiodide-containing elements including DIRs, providing materials with good sharpness and superior shelf stability. U.S. Pat. No. 5,356,764 discloses a tabular grain silver halide emulsion layer including an image coupler and a DIR coupler wherein at least 50 mol % of the total projected grain area is accounted for by specific tabular grains containing at least 50 mol % silver chloride, providing elements with acceptable image sharpness and reduced gamma (contrast). However there can be some loss in speed and reduced latitude associated with non-linear sensitometric curves and undesirable xe2x80x98toe scoopingxe2x80x99.
In order for the DIR to be able to compete effectively with the image coupler for oxidised developer it has been the practice for it to be more reactive (typically 5-10xc3x97) than its associated image coupler, particularly in bromoiodide films, such as described in, for example, U.S. Pat. Nos. 4,286,054, 4,315,070, 4,387,159, 4,348,474 and 4,980,267.
While DIR compounds provide advantageous image properties with silver bromoiodide emulsions, proper development without side-effects is much more difficult to achieve for silver bromide or chloride emulsions because of their superior developability. Silver chloride emulsions can lead to faster and easier processing, including faster and easier development, bleaching and fixing, combined with lower environmental impact.
There has been a need to provide a means of effectively reducing contrast in elements containing silver halide emulsions comprising at least 50 mol % silver chloride, whereby a linear sensitometric curve over a good latitude with little or no xe2x80x98toe scoopingxe2x80x99 or speed loss is obtained.
It has now been found that the DIR couplers used in elements according to this invention which have a low reactivity relative to its associated image coupler can reduce the contrast of silver halide emulsions comprising at least 50 mol % silver chloride more effectively than DIR couplers of high reactivity, with significant advantages being observed in the sensitometric curve.
According to the present invention therefore there is provided a photographic element comprising at least one emulsion layer comprising at least 50 mol % silver chloride, which layer comprises at least one DIR coupler in association with at least one image dye-forming coupler, characterised in that the relative reactivity ratio krel of at least one DIR coupler and an associated image coupler is less than or equal to 1.0, wherein
krel=k1/k2;
k1=the second order rate constant for the reaction of DIR coupler with oxidised developer and
k2=the second order rate constant for the reaction of image coupler with oxidised developer.
In another aspect of the invention there is provided a multicolour photographic element comprising a support bearing a cyan image-dye-forming unit comprising at least one red-sensitive silver halide emulsion layer and a cyan dye-forming coupler; a magenta image-dye-forming unit comprising at least one green-sensitive silver halide emulsion layer and a magenta dye-forming coupler; a yellow image-dye-forming unit comprising at least one blue-sensitive silver halide layer and a yellow dye-forming coupler, wherein at least one of the layers comprises an emulsion comprising at least 50 mol % silver chloride, which layer comprises at least one DIR coupler in association with at least one image dye-forming coupler, characterised in that in any one layer the relative reactivity ratio krel of at least one DIR coupler and an associated image coupler is less than or equal to 1.0, where krel is as hereinbefore defined.
In yet another embodiment of the invention there is provided a process of forming an image in a photographic element as hereinbefore defined after the element has been imagewise exposed to light, comprising contacting the element, as herein described, with a colour developing agent.
The DIR couplers, when associated with image couplers in elements of the invention, efficiently reduce development of silver halide emulsions containing at least 50 mol % silver chloride, i.e. there is a reduction in contrast, providing a linear sensitometric curve over a good latitude with negligible speed penalty.